The present invention relates to apparatus and methods for processing substrates such as semiconductor substrates for use in IC fabrication or glass panels for use in flat panel display applications. More particularly, the present invention relates to controlling a plasma inside a plasma process chamber.
Plasma processing systems have been around for some time. Over the years, plasma processing systems utilizing inductively coupled plasma sources, electron cyclotron resonance (ECR) sources, capacitive sources, and the like, have been introduced and employed to various degrees to process semiconductor substrates and glass panels.
During processing, multiple deposition and/or etching steps are typically employed. During deposition, materials are deposited onto a substrate surface (such as the surface of a glass panel or a wafer). For example, deposited layers such as SiO.sub.2 may be formed on the surface of the substrate. Conversely, etching may be employed to selectively remove materials from predefined areas on the substrate surface. For example, etched features such as vias, contacts, or trenches may be formed in the layers of the substrate.
One particular method of plasma processing uses an inductive source to generate the plasma. FIG. 1 illustrates a prior art inductive plasma processing reactor 100 that is used for plasma processing. A typical inductive plasma processing reactor includes a chamber 102 with an antenna or inductive coil 104 disposed above a dielectric window 106. Typically, antenna 104 is operatively coupled to a first RF power source 108. Furthermore, a gas port 110 is provided within chamber 102 that is arranged for releasing gaseous source materials, e.g., the etchant source gases, into the RF-induced plasma region between dielectric window 106 and a substrate 112. Substrate 112 is introduced into chamber 102 and disposed on a chuck 114, which generally acts as a bottom electrode and is operatively coupled to a second RF power source 116.
In order to create a plasma, a process gas is input into chamber 102 through gas port 110. Power is then supplied to inductive coil 104 using first RF power source 108. The supplied RF energy passes through dielectric window 106 and a large electric field is induced inside chamber 102. The electric field accelerates the small number of electrons present inside the chamber causing them to collide with the gas molecules of the process gas. These collisions result in ionization and initiation of a discharge or plasma 118. As is well known in the art, the neutral gas molecules of the process gas when subjected to these strong electric fields lose electrons, and leave behind positively charged ions. As a result, positively charged ions, negatively charged electrons and neutral gas molecules (and/or atoms) are contained inside the plasma 118.
Once the plasma has been formed, neutral gas molecules inside the plasma tend to be directed towards the surface of the substrate. By way of example, one of the mechanism contributing to the presence of the neutrals gas molecules at the substrate may be diffusion (i.e., the random movement of molecules inside the chamber). Thus, a layer of neutral species (e.g., neutral gas molecules) may typically be found along the surface of substrate 112. Correspondingly, when bottom electrode 114 is powered, ions tend to accelerate towards the substrate where they, in combination with neutral species, activate the etching reaction.
Plasma 118 predominantly stays in the upper region of the chamber (e.g., active region), however, portions of the plasma tend to fill the entire chamber. The plasma typically goes where it can be sustained, which is almost everywhere in the chamber. By way of example, the plasma may fill the areas below the substrate such as the bellows of the pumping arrangement (e.g., non-active region). If the plasma reaches these areas, etch, deposition and/or corrosion of the areas may ensue, which may lead to particle contamination inside the process chamber, i.e., by etching the area or flaking of deposited material. Accordingly, the lifetime of the chamber parts is typically reduced.
Furthermore, an unconfined plasma tends to form a non uniform plasma, which may lead to variations in the process performance, i.e. etch uniformity, overall etch rate, etch profile, micro-loading, selectivity, and the like. As a result, it is extremely difficult to control the critical dimensions of the integrated circuit. Additionally, variations in the process performance may lead to device failure in the semiconductor circuit, which typically translates into higher costs for the manufacturer.
The standard solution for controlling the plasma has been to provide a plasma screen inside the plasma reactor. The plasma screen is generally dimensioned to confine the plasma within a volume defined by the process chamber and the plasma screen. In most cases, the plasma screen also includes a plurality of openings for permitting by-product gases formed during processing to pass through to the exhaust ports of the plasma reactor.
Referring to FIGS. 1 & 2, a plasma screen 202 is shown in conjunction with plasma processing chamber 100. Plasma screen 202 is typically configured to substantially fill the gap made between the inside periphery of a chamber wall 120 and the outer periphery of electrostatic chuck 114. Further, plasma screen 202 typically includes a plurality of perforations 204 that are dimensioned to allow the by-product gas, formed during processing, to escape and be exhausted out of an exhaust port 122. At the same time, perforations 204 are dimensioned to confine the plasma to the volume defined by process chamber 102. The perforations are generally patterned to be circular, slotted, concentric and/or the like. Further still, the plasma screen is typically attached (e.g., bolted) to the chamber in a fixed position.
However, the plasma screen has some drawbacks. Typically, structures that are disposed inside the processing chamber during processing tend to cause contamination of the substrate. This is because such structures may present sites or surfaces for adsorbed materials to attach, for example, etch by-products and deposits, which may flake off onto the substrate causing particle contamination. Particle contamination may produce undesirable and/or unpredictable results. For example, particles on the substrate surface may block a portion of the substrate that needs to be etched. In this manner, a trench may not be formed properly and this may lead to device failure and therefore a reduction in productivity. Further, the plasma screen has to be cleaned at various times during processing to prevent excessive build-ups of deposits and etched by-products. Cleaning disadvantageously lowers substrate throughput, and typically adds costs due to loss of production.
Additionally, the plasma screen reduces the conductance path for the by-product gases. By way of example, the plasma screen typically reduces the conductance path of by-product gas from 30% to 60%. This tends to increase the demand on the pumping arrangement. That is, a larger turbo-molecular pump has to be used in order to effectively remove the byproduct gases and maintain the desired chamber pressure through the reduced conductance.
Moreover, the perforations may get clogged during processing, which may further reduce the conductance. Again, a loss of conductance may adversely effect the proper functioning of the pumping system, i.e., reduce the flow. This may lead to process variation, and reduce the life of the pump, which further reduces productivity and typically adds cost. Further still, plasma screens tend to be consumable items because they are in contact with the plasma and therefore tend to be bombarded by the reactive species in the plasma.
In addition, bolting the plasma screen to the chamber typically limits the types of materials that can be used without breaking during normal installation. Further still, the electrical and thermal contact between the plasma screen and the chamber can be difficult to ensure.
In view of the foregoing, there are desired improved techniques and apparatuses for controlling the volume of a plasma inside a process chamber.